this is common to a lot of species that trawl around the floor picking at gravel. the fish like to pick at it looking for food and sometimes they get a piece thats too big to spit out.
the best course of action is to leave the fish as long as possible in the hope they will throw it out. if the fish starts to struggle then action is needed. usually its a big fish that this happens to so the best thing to do is gently net the fish and then wrap it in a wet flannel. try and cover the eyes too to avoid stress. next, gently tip the fish (over the tank) and see if gravity will help move it. if not, then gently use your finger and press on the chin to make the fish open up. you should be able to see the offending piece of gravel. tip the fish over again and stroke the throat toward the opening of the mouth, this sometimes moves the stone so the fish can release it. if all this fails then you will need a pair of tweezers to gently grab the gravel and pull gently forwards and be careful so as not to rip the throat. the fish can be out of water for approximately 1 minute between attempts and should be placed back in the water at regular intervals to get oxygen.
to avoid this from happening, the gravel needs to be a lot smaller than the mouth or too big to be picked up.
hope this helps.
